Pressure sensor having metallic diaphragm seal joint

ABSTRACT

A pressure sensor that is particularly suitable inter alia for the food industry and the measuring accuracy of which is stable over a long time, having a diaphragm seal ( 3, 45 ) with a separating diaphragm ( 7, 53 ) on which a pressure (P) to be measured acts and having a ceramic measuring cell ( 9, 57 ) which is connected to the diaphragm seal ( 3, 45 ) exclusively by inorganic materials, is provided, in which sensor the separating diaphragm ( 7, 53 ) and all further sensor components coming into contact during measurement with a medium of which the pressure (P) is to be measured are metallic.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.09/734,739 filed Dec. 13, 2000, now U.S. Pat. No. 6,715,356.

FIELD OF THE INVENTION

The invention relates to a pressure sensor.

BACKGROUND OF THE INVENTION

In pressure measurement technology, absolute—and relative-pressuresensors are used, for example. In the case of absolute-pressure sensors,a pressure to be measured is registered in absolute terms, i.e. as apressure difference with respect to a vacuum. With a relative-pressuresensor, a pressure to be measured is picked up in the form of a pressuredifference with respect to a reference pressure, for example a pressurewhich prevails where the sensor is located. In most applications, thisis the atmospheric pressure at the place of use. Consequently, in thecase of the absolute-pressure sensor a pressure to be measured is sensedin relation to a fixed reference pressure, the vacuum pressure, and inthe case of the relative-pressure sensor a pressure to be measured isregistered in relation to a variable reference pressure, for exampleambient pressure.

Ceramic pressure-measuring cells are advantageously used in pressuremeasurement technology, since ceramic pressure-measuring cells have ameasuring accuracy which is stable over a very long time. One reason forthis is the solid ionic bonding of ceramic, which makes the materialvery durable and undergo virtually no ageing in comparison with othermaterials, for example metals. However, in comparison with metal,ceramic pressure sensors have a rougher surface and are often restrainedby means of a generally nonreplaceble seal made of an organic material,for example an elastomer, in a pressure-tight manner in a housing whichcan then fastened at a measuring location by means of a processconnection.

In the food industry, pressure sensors which can be fitted such thatthey are flush at the front are used with preference, all sensorelements coming into contact with a medium of which the pressure is tobe measured consisting of a metal, preferably of a stainless steel whichcan be cleaned very well.

In this branch of industry it is additionally of particular advantage ifthe pressure sensors have as few seals as possible. Seals consist oforganic materials and, for reasons of hygiene, should thereforepreferably be replaceable. In an ideal case, there is just a single sealfor sealing off the process connection. In contrast to a seal belongingto the sensor, this seal, referred to hereafter as the process seal, canbe exchanged at any time by the user himself without any problem, inparticular without any effect on the measuring accuracy of the pressuresensor.

SUMMARY OF THE INVENTION

It is an object of the invention to specify a pressure sensor which issuitable inter alia for the food industry and the measuring accuracy ofwhich is stable over a long time.

For this purpose, the invention comprises a pressure sensor having

-   -   a diaphragm seal with a separating diaphragm, on which a        pressure to be measured acts, and    -   a ceramic measuring cell, connected exclusively by inorganic        materials to the diaphragm seal,    -   in which the separating diaphragm and all further sensor        components coming into contact during measurement with a medium        where pressure is to be measured are metallic.

According to one development, the measuring cell is fixed withoutrestraint in a housing by being seated in the axial direction on a smalltube, via which the ceramic measuring cell is connected to the diaphragmseal.

According to one development, the measuring cell has a measuringdiaphragm, which subdivides an interior space of the measuring cell intoa first chamber and a second chamber. The first chamber is connected tothe diaphragm seal via a small tube, the first chamber, the small tubeand the diaphragm seal are filled with a fluid, the fluid transfers apressure acting on the separating diaphragm to the measuring diaphragm,a reference pressure in the second chamber acts on the measuringdiaphragm, and the pressure sensor has an electromechanical transducerfor registering a deflection of the measuring diaphragm dependent on thepressure and the reference pressure and for converting said deflectioninto an electrical output signal.

According to one embodiment, the reference pressure is a referencepressure prevailing in the ambience and the second chamber has anopening through which the reference pressure is introduced into thesecond chamber, or the second chamber is hermetically sealed and thereference pressure is an absolute pressure prevailing in the secondchamber.

According to one embodiment, the measuring cell is additionally enclosedin the radial direction in a holder.

According to one embodiment, the holder has a body made of an elastomer,filling an intermediate space between the measuring cell and thehousing.

According to another development, the ceramic measuring cell is fastenedin a housing connected to the diaphragm seal. In this case, the housingpreferably consists of a material which has a coefficient of thermalexpansion which is approximately equal to the coefficient of thermalexpansion of the ceramic of the measuring cell.

According to one development, the measuring cell is fastened in aninsert which is arranged in a housing, is connected to the diaphragmseal and reaches around the measuring cell in a pot-like manner.According to a further development of the invention, the measuring cellis mounted without restraint and isostatically in a chamber filled witha fluid, surrounded on all sides by the fluid.

Investigations have shown that, in the case of a ceramic measuring cellrestrained in a pressure-tight conventional way by means of an organicmaterial, for example a seal made of an elastomer, diaphragm sealscannot be used without sacrificing considerable measuring accuracy.Changes in temperature and/or pressure can cause positional and/ordimensional changes of the seal, which are accompanied by a displacementof diaphragm seal fluid. In the case of a diaphragm seal, only a smallamount of the diaphragm seal fluid is displaced when there is a changein pressure. If there are seal-related volume displacements of the sameorder of magnitude as pressure-related volume displacements, meaningfulmeasurement is no longer possible.

Customarily used sealing materials are plastics, such aspolytetrafluoroethylene or Viton for example. These materials are notgastight. If a negative pressure acts on the pressure sensor, air or gascan diffuse into the diaphragm-seal fluid through the seal from a sideof the pressure sensor facing away from the diaphragm seal. Air or gasin the diaphragm-seal fluid greatly impairs the measuring accuracy ofthe pressure sensor.

On account of the way in which the connection of the diaphragm seal ismade according to the invention, using connections made of inorganicmaterials, seals can be dispensed with completely. It is consequentlypossible for the first time to use a ceramic pressure-measuring cell incell in connection with a diaphragm seal and to utilize the advantagesof ceramic measuring cells, that is their stable measuring accuracy overa very long time, in connection with a diaphragm seal.

Mechanical connections of inorganic materials may be, for example,welded or soldered connections, in particular active brazed connections.Such metallic joints offer the advantage that they are gastight and, incomparison with methods of connection by means of organic materials,such as by means of restrained seals for example, are mechanicallyimmovable and to the greatest extent free from creepage. Consequently,in the case of a pressure sensor according to the invention, changes inpressure and/or temperature do not cause permanent deformation of theconnecting materials at the connection points that could lead to adeterioration in the measuring accuracy. The measuring accuracy of thepressure sensors according to the invention can therefore be guaranteedover very long time periods.

A further advantage is that, in spite of the use of a ceramicpressure-measuring cell, apart from the process seal, only metallicmaterials come into contact with the medium of which the pressure is tobe measured. Use of the diaphragm seal makes it possible for the metalcontacted by the medium to be freely selected within wide limits,according to the mechanical and/or chemical properties of the medium.

There are also applications in which the diaphragm seal is fitteddirectly at the measuring location in a so-called welded neck, i.e. aneck welded onto the container. In welded necks, the sealing usuallytakes place purely metallically, for example by means of sealing cones.In these cases, even the process seal is omitted.

The invention and further advantages are now explained in more detailwith reference to the figures of the drawing, in which four exemplaryembodiments are represented. The same elements are provided with thesame reference numerals in the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a section through a pressure sensor according to theinvention, in which a ceramic measuring cell is fixed on a small tube,by means of which it is connected to a diaphragm seal;

FIG. 2 shows an enlarged representation of the measuring cell from FIG.1;

FIG. 3 shows a section through a further pressure sensor according tothe invention, in which a ceramic measuring cell is fastened in ahousing;

FIG. 4 shows a partially sectioned view of a pressure sensor, in which aceramic measuring cell is arranged in an insert in a housing;

FIG. 5 shows an enlargement of the lead-through of a small tube filledwith fluid through the insert from FIG. 4;

FIG. 6 shows an enlargement of the joining location between the housingand the measuring cell from FIG. 4; and

FIG. 7 shows a pressure sensor in which the measuring cell is mounted ina chamber filled with a fluid.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, a section through a pressure sensor according to theinvention is represented. The pressure sensor comprises an essentiallycylindrical housing 1, in the one end of which a diaphragm seal 3 isenclosed. It is welded into the housing 1. The diaphragm seal 3 has achamber 5, which is filled with a fluid and closed by a separatingdiaphragm 7. During operation, a pressure P to be measured, which isindicated in FIG. 1 by an arrow, acts on the separating diaphragm 7.

Arranged in the housing 1 is a ceramic measuring cell 9, which isconnected to the chamber 5 of the diaphragm seal 3 via a small tube 11filled with the fluid.

FIG. 2 shows an enlarged representation of the measuring cell 9. Itcomprises two cylindrical basic bodies 13 and a measuring diaphragm 15enclosed between the two basic bodies.

The measuring cell 9 is a ceramic measuring cell, i.e. the basic bodies13 and the measuring diaphragm 15 consist of ceramic. The measuringdiaphragm 15 is connected in a pressure-tight and gastight manner toeach of the basic bodies 1, at its edge facing the respective basic body13, by means of a joining location 14. Suitable for example as thejoining material is an active brazing solder. The measuring diaphragm 15is pressure-sensitive, i.e. a pressure acting on it causes a deflectionof the measuring diaphragm 15 from its position of rest.

The measuring diaphragm 15 and the joining locations 14 subdivide aninterior space of the measuring cell 9 into a first chamber 17 and asecond chamber 19. The ceramic measuring cell 9 is connected to thediaphragm seal 3 exclusively by inorganic materials. Suitable forexample for this are connecting or joining techniques such as solderingor welding. In the exemplary embodiment represented in FIG. 1, the firstchamber 17 is connected to the diaphragm seal 3 via the small tube 11.The small tube 11 is, for example, welded onto the diaphragm seal 3 andfastened to the basic body 13 by a soldered connection. No seal of anorganic material is required.

Just like the diaphragm seal 3 and the small tube 11, the first chamber17 is filled with fluid. A pressure P acting on the separating diaphragm7 is transferred by the fluid to the measuring cell 3 into the firstchamber 17. The fluid is as incompressible as possible and has acoefficient of thermal expansion that is as low as possible. Suitablefor example are commercially available silicone oils. In addition, thefilling amount required is preferably to be kept low, by the small tube11 having a small diameter and the diaphragm seal 3 in the chamber 5having a diaphragm bed which mimics the shape of the separatingdiaphragm 7 and is arranged at a small distance from the separatingdiaphragm 7.

For the case in winch the pressure sensor is to be used in hazardouslocations where there is a risk of explosion, a flame barrier may bearranged in the small tube 11 or the small tube 11 may itself bedimensioned in such a way as to form a flame barrier. The constructionof such a flame barrier can be taken from national safety regulationsand standards on explosion protection.

In the exemplary embodiment represented in FIG. 1, the second chamber 19has an opening 21, in this case a bore penetrating the basic body 13,through which a reference pressure is introduced into he second chamber19. The reference pressure is reference pressure P_(R) prevailing in thepressure sensor, in this case an ambient pressure. This is thus arelative-pressure sensor.

Instead of the ambient pressure, a variable pressure may also beintroduced into the second charmer 19, for example via a seconddiaphragm seal, connected in an analogous way to the diaphragm seal 3.In this case, the deflection of the measuring diaphragm is dependent onthe difference between the two pressures acting on it.

The pressure sensor according to the invention may of course also bedesigned as an absolute-pressure sensor. In this case, the secondchamber 19 is evacuated and hermetically sealed and the referencepressure is an absolute pressure prevailing in the second chamber 19.The measuring cell 9 has an electromechanical transducer for registeringa deflection of the measuring diaphragm 15 dependent on the pressure Pand the reference pressure and for converting said deflection into anelectrical output signal.

In the exemplary embodiment represented in FIGS. 1 and 2, theelectromechanical transducer comprises a capacitor, which has ameasuring electrode 23, arranged in the first chamber 17 on themeasuring diaphragm 15, and a counterelectrode 25, arranged opposite themeasuring electrode 23 on an inside wall of the first chamber 17, on thebasic body 13. The capacitance of the capacitor depends on the distanceof the measuring electrode 23 and the counterelectrode 25 in relation toone another and is consequently a measure of the deflection of themeasuring diaphragm 15.

The measuring electrode 23 is electrically contacted through the joininglocation 14 and is connected outside, for example to ground. Thecounterelectrode 25 is electrically contacted through the basic body 13,to the outer side of the latter, and leads to an electronic circuit 27arranged on the basic body 13. Measuring electrode 23 andcounterelectrode 25 form a capacitor and the electronic circuit 27converts the changes in capacitance of the capacitor, for example into acorrespondingly changing electrical voltage. The output signal isavailable for further processing and/or evaluation via connecting leads29.

If the pressure sensor is to be used at very high temperatures, it isrecommendable to arrange the electronic circuit 27 at some distance fromthe diaphragm seal 3 and the ceramic measuring cell 9. It is alsopossible of course for more electrodes to be arranged in the firstchamber 17, on the basic body 13 and/or on the measuring diaphragm 15.For example, a circular-disk-shaped inner electrode and an outerannular-disk-shaped electrode surrounding the latter may be providedinstead of the counterelectrode 25.

The outer electrode together with the measuring electrode 23 would forma second capacitor, the capacitance of which may serve for compensatingpurposes, while the inner electrode together with the measuringelectrode 23 has a capacitance dependent on the pressure and on thereference pressure.

However, piezoresistive elements or strain gauges arranged on themeasuring diaphragm 15 in the first chamber 17 can also be used aselectromechanical transducers.

A great advantage of the pressure sensor described above when designedas a relative-pressure sensor is that the electromechanical transduceris completely protected against moisture, for example condensate, andcontaminants. Moisture and/or contaminants, as are typically containedin the atmosphere and in the pressure sensor, can be deposited only inthe second chamber 19. By contrast, the first chamber 17, which containsthe electromechanical transducers sensitive to moisture and/orcontaminants, is closed from the environment.

The measuring cell 9 is fixed in the housing 1, by being seated in theaxial direction on the small tube 11, via which the ceramic measuringcell 9 is connected to the diaphragm seal 3. In addition, it is enclosedin the radial direction in a holder. Pressure-resistant restraint, as isrequired in the case of conventional ceramic pressure-measuring cells,is not necessary in the case of the pressure sensor according to theinvention, since the pressure P is introduced through the diaphragm sealinto the first chamber 17 and consequently exerts only a very smalloverall force on the measuring cell 9 via the thin small tube 11. Arestraint generally causes a reaction, in particular a pressure- andtemperature-dependent reaction, on the measuring cell.

In particular if the restraint causes a reaction on the measuringdiaphragm, this may lead to a change in sensor data of the pressuresensor, for example its zero point or its characteristic temperaturedata and thus to measurement errors.

The fitting of the ceramic measuring cell 9 without restraint has theeffect of improving still further the measuring accuracy of ceramicmeasuring cells, which is in any case very stable over a long time incomparison with other measuring cells.

In the case of the exemplary embodiment represented in FIG. 1, there isformed onto the housing 1, at the level of the measuring cell 9, aradially inwardly extending shoulder 31, on the inner circumferentialsurface of which there is arranged a spring 33 running around theperiphery in an annular form. An intermediate space existing between thehousing 1 and measuring cell 9 is filled by a body 35 made of elastomer.The body 35 has a groove which runs around the outside periphery in anannular form and into which the spring 33 of the shoulder 31 looselyengages. The body 35 reaches around the measuring cell 9 and prevents adeflection in the radial direction of the measuring cell 9 seated on thesmall tube 11. In the axial direction, on the other hand, the measuringcell 9 is movable, in order to be able to compensate for differences inthermal expansion.

The housing 1 is adjoined in the direction away from the diaphragm sealby a connection housing 37. In the exemplary embodiment shown, theconnection housing 37 is screwed onto the housing 1. Arranged in theconnection housing 37 there are, for example, continuing electronics,which are not represented in FIG. 1 and in which the measuring signalsare preprocessed. The shoulder 31 has at the side a bore 39, throughwhich the connecting leads 29 are led. The measuring signals areaccessible for further processing and/or evaluation via the connectingleads 29.

On a side facing away from the measuring cell 9, the diaphragm seal 3 isdesigned as a process connection 41. The process connection 41 servesthe purpose of fastening the pressure sensor at a measuring location. Inthe exemplary embodiment shown, the process connection 41 is a standardconnection, as defined in the international standard ISO 2852. Thisconnection is known in measuring technology by the trade name‘Triclamp’. Other types of fastening can likewise be used. Theseparating diaphragm 7 terminates flush at the front with the processconnection 41 and forms a pressure—and gastight termination with respectto the process. Other types of fastening, for example by means of aflanged or screwed connection, can likewise be used.

A pressure P prevailing at the measuring location acts directly on theseparating diaphragm 7 and is transferred via the diaphragm seal 3 andthe fluid in the small tube 11 into the measuring cell 9.

The separating diaphragm 7 and all further sensor components coming intocontact during measurement with a medium whose pressure is to bemeasured, in the exemplary embodiment shown i.e. the process connection41, are metallic.

Metal offers the treat advantage here that such a sensor can be fittedsuch that it is flush with the front and consequently can be cleanedwell.

The pressure sensor according to the invention offers the advantage thatthe pressure sensor itself manages completely without a seal coming intocontact with the medium. Only a single seal, that is a process seal forsealing off the measuring location from the ambience, is required. Theprocess connection 41 has a groove 42 running around the periphery in anannular form for receiving this process seal, which is not representedin FIG. 1. The process seal can be exchanged at any time without anyproblem and an exchange of the process seal has no influence on themeasuring accuracy of the pressure sensor whatsoever.

In some applications, it is possible to fit the pressure sensoraccording to the invention directly in a so-called welded neck, i.e. aneck welded onto the container. In welded necks, sealing usually takesplace purely metallically, for example by means of sealing cones. Thisoffers the advantage over the process connection 41 represented that thepressure sensor is not only arranged such that it is flush at the frontbut also manages entirely without seals, even without a process seal.

The pressure sensor is therefore very good for applications in the foodindustry, where the requirements for cleanability, freedom from sealsand for metallic materials are given particularly high importance.

The measuring cell 9 represented in FIGS. 1 and 2 is a relative-pressuremeasuring cell. The pressure P to be measured is registered in relationto the reference pressure, the reference pressure in this case being thevariable ambient pressure. An absolute-pressure sensor may also beconstructed in an entirely analogous way to the relative-pressure sensordescribed above. In the case of such an absolute-pressure sensor, theopening 21 is omitted, and the second chamber 19 is evacuated. In acorresponding way, the reference pressure is then the fixed vacuumpressure in the second chamber.

FIG. 3 shows a section through a second exemplary embodiment of apressure sensor according to the invention. It has a metallic diaphragmseal 45, adjacent to which there is a housing 47 and a connectionhousing 49, connected to the housing 47.

The diaphragm seal 45 is essentially cylindrical and has at the end achamber 51, which is filled with a fluid and closed by a metallicseparating diaphragm 53. The diaphragm seal 45 and the separatingdiaphragm 53 preferably consist of a high-grade and corrosion-resistantstainless steel. During operation, a pressure P to be measured, which isindicated in FIG. 3 by an arrow, acts on the separating diaphragm 53.

The housing 47 is cylindrical and rests with a circular-annular basesurface on a circular-annular end face of the diaphragm seal 45 facingaway from the separating diaphragm. Housing 47 and diaphragm seal 45 areeither a single component or are connected to one another by aconnection 55 made of an inorganic material.

Arranged in the housing 47 is a ceramic measuring cell 57. In theexemplary embodiment shown, the measuring cell 57 is a capacitiveceramic absolute-pressure measuring cell. Alternatively, arelative-pressure measuring cell may of course also be used. Themeasuring cell 57 has a basic body 59 and a measuring diaphragm 61. Thebasic body 59 and the measuring diaphragm 61 consist of ceramic. Themeasuring diaphragm 61 and the basic body 59 are connected in apressure-tight and gastight manner to one another at their edge by meansof a joining location, thereby forming a measuring chamber. Themeasuring diaphragm 61 is pressure-sensitive, i.e. a pressure acting onit causes a deflection of the measuring diaphragm 61 from its positionof rest.

Arranged on one inner side of the measuring diaphragm 61 is an electrode63 and arranged on an opposite inner side of the basic body 59 is atleast one counterelectrode 65. The electrode 63 of the measuringdiaphragm 61 is electrically contacted by the joining location and isconnected on the outside, for example, to ground. The counterelectrode65 of the basic body 59 is electrically contacted through the basic body59 to the outer side of the latter and leads to an electronic circuit 67arranged on the basic body 59. Electrode 63 and counterelectrode 65 forma capacitor, and the electronic circuit 67 converts the changes incapacitance of the capacitor, for example into a correspondinglychanging electrical voltage. The measured variable is available forfurther processing and/or evaluation via connecting leads 69.

The ceramic measuring cell 57 is fastened in the housing 47 by means ofa connection made of an inorganic material 71. In the exemplaryembodiment shown, the ceramic measuring cell 57 and the housing 47 arecylindrical and arranged coaxially in relation to one another, so thatthe housing 47 surrounds the measuring cell 57. The connection 71 ispreferably arranged in/an annular-cylindrical gap between the housing 47and the basic body 59. This achieves the effect that the sensitivemeasuring diaphragm 61 remains essentially free from restraint.

The housing 47 preferably consists of a material which has a coefficientof thermal expansion which is approximately equal to the coefficient ofthermal expansion of the ceramic of the measuring cell 57. If, forexample, a measuring cell made of an aluminum oxide is used,nickel-iron-cobalt alloys, as are commercially available for exampleunder the Product name Vacon or Kovar, are suitable materials for thehousing 47. Alternatively, the housing 47 may, however, also likewiseconsist of a ceramic, for example an aluminum oxide. Such a suitablechoice of material achieves the effect that only very low forces areexerted on the measuring cell 57 by the housing 47, even when there arestrong fluctuations in temperature.

In the case of a metallic housing 47, the connection 55 may be, forexample, a welded connection and the connection 71 may be, for example,a metallic joint, for example with an active brazing solder. In the caseof a housing 47 made of ceramic, the connections 55 and 71 may bemetallic joints, for example with an active brazing solder. The ceramicmeasuring cell 57 is connected to the diaphragm seal 45 via theconnections 55 and 71 exclusively by inorganic materials. In the case ofthis exemplary embodiment as well, the pressure sensor itself has noseals whatsoever. All that is required is a single process seal, whichcan be exchanged without any effects on the measuring accuracy and isnot represented in FIG. 2.

The diaphragm seal 45 has a through-bore 73, the one end of which opensout into the chamber 51 and the other end of which opens out in achamber 75 bounded by the housing 47 and the measuring cell 9. Thechamber 51, the bore 73 and the chamber 75, as well as an interior spaceof the housing 47, facing the diaphragm seal 45 in front of themeasuring cell 57, are filled with a fluid that is as incompressible aspossible, for example a silicone oil.

By means of the bore 73 and the metallic connections between thediaphragm seal 45 and the housing 47, as well as between the housing 47and the measuring cell 57, the ceramic measuring cell 57 is connected tothe diaphragm seal 45 by inorganic materials. A pressure P acting on theseparating diaphragm 53 is transferred by the fluid to the measuringdiaphragm 61, and a deflection of the measuring diaphragm 61 dependenton the absolute pressure to be measured is registered by the capacitiveelectromechanical transducer described above and converted by theelectronic circuit 67 into an electrical measured variable.

Instead of the capacitive ceramic measuring cell described, apiezoresistive measuring cell may also be used, for example. In the caseof these types of measuring cells, the transducer has strain gaugesapplied to the measuring diaphragm. In the case of these measuringcells, the measuring chamber may also be formed by the basic body, onwhich the measuring diaphragm is fastened by its outer edge, and themeasuring diaphragm itself.

Of course, a relative-pressure measuring cell may also be used insteadof the absolute-pressure measuring cell described.

The diaphragm seal 45 has at its end facing away from the measuring cellan external thread 79, by means of which the pressure sensor can bescrewed at a measuring location into a corresponding opening such thatit is flush at the front. The diaphragm seal 45 consequently serves atthe same time as a process connection. Above the external thread 79, thediaphragm seal 45 has an annular shoulder surface, in which a peripheralgroove 78 for receiving the process seal is provided.

In the case of the exemplary embodiment represented in FIG. 3 as well,the separating diaphragm 53 and all further sensor components cominginto contact during measurement with a medium of which the pressure P isto be measured, in this case only the external thread 79, are metallic.

FIG. 4 shows a partially sectioned view of a further exemplaryembodiment of a pressure sensor according to the invention. The pressuresensor has a diaphragm seal 45 with a separating diaphragm 7, on which apressure P to be measured acts, which seal is identical to the diaphragmseal 45 represented in FIG. 3. The separating diaphragm 7 and allfurther sensor components coming into contact during measurement with amedium of which the pressure P is to be measured, in this case theexternal thread 79, are metallic.

Formed onto the diaphragm seal 45 in the direction away from theseparating diaphragm is a housing 81, which has a cylindrical interiorspace. Arranged in housing 81 is a pot-shaped insert 83, which isconnected to the diaphragm seal 45 and serves for receiving the ceramicmeasuring cell 57. In the exemplary embodiment shown, the measuring cell57 corresponds to the measuring cell 57 represented in FIG. 3. The otherceramic measuring cells can of course also be used.

The insert 83 reaches around the ceramic measuring cell 57 in a pot-likemanner. In this case, the measuring diaphragm 61 rests with an outerpressure-insensitive edge on a narrow shoulder ring 85, which runsaround the inside periphery of the insert 83 and can be seen in theenlargement represented in FIG. 6.

The insert 83 preferably consists of a material of which the coefficientof thermal expansion is approximately equal to the coefficient ofthermal expansion of the ceramic. Suitable materials are, for example,ceramic or iron-nickel-cobalt alloys, as are available under she tradename Vacon or Kovar.

The ceramic measuring cell 57 is connected at an outer cylindrical edge,facing away from the measuring diaphragm, to the insert 83 by means ofan inorganic material. This may take place, for example, by a solderedconnection. FIG. 6 shows an enlargement of the connecting point betweenthe housing and the measuring cell 57 from FIG. 4. A solder ring 87,which is introduced into a recess between the insert 83 and themeasuring cell 57, is represented. During the soldering operation, thesolder becomes liquid and fills a narrow annular-cylindrical gapexisting between the measuring cell 57 and the insert 83.

The diaphragm seal has a bore 73, into which a small tube 89 isadmitted. The small tube 89 is continued on a side of the diaphragm seal45 facing away from the separating diaphragm and is led through theinsert 83.

At its end facing the separating diaphragm, the small tube 89 is weldedinto the diaphragm seal 45. The insert 83 is fastened on one end of thesmall tube 89, facing away from the separating diaphragm, to minimizemechanical stresses, as can occur on account of different coefficientsof expansion of the housing 81, insert 83 and measuring cell 57. Thereis preferably not a mechanically fixed connection between the insert 83and the housing 81.

The small tube 89, the chamber of the diaphragm seal and a hollow space,surrounding the measuring cell 57 in the insert 83, are fluid-filled.The fluid transfers a pressure P acting on the separating diaphragm to ameasuring diaphragm of the ceramic measuring cell 57.

FIG. 5 shows an enlargement of the lead-through of the small tube 89through the insert 83. The small tube 89 preferably consists of aniron-nickel-cobalt alloy, for example made of one of the materialsalready mentioned Vacon or Kovar, and is led through the insert 83 in aceramic ring 91 metallized on both sides. The ceramic ring 91 causes anelectrical insulation between the housing 81 and the insert 83. Theceramic ring 91 is, for example, connected to the insert 83 and thesmall tube 89 by soldering.

The ceramic measuring cell 57 is connected to the diaphragm seal 45 viathe small tube 99 and the connection between the measuring cell 57 andthe insert 83 exclusively by means of inorganic materials.

FIG. 7 shows a partially sectioned view of a further exemplaryembodiment of a pressure sensor according to the invention. In the caseof this exemplary embodiment as well, the diaphragm seal 45 and theceramic measuring cell 57 are designed essentially according to theexemplary embodiments represented in FIGS. 3 and 4.

Formed onto the diaphragm seal 45 is a cylindrical housing 93, which isterminated by a cover 95. Housing 93 and cover 95 preferably consist ofa metal and are connected to one another by a welded connection 97. Theinterior space of the housing 93 forms a chamber 98, in which theceramic measuring cell 57 is arranged. Here, too, the diaphragm seal 45has a through-bore 73, through which the chamber 98 is connected to thechamber of the diaphragm seal 45 lying behind the separating diaphragm7. The chamber of the diaphragm seal 45, the bore 73 and the chamber 98in which the measuring cell 57 is arranged are filled with a fluid thatis as incompressible as possible and has a coefficient of thermalexpansion that is as low as possible, for example a silicone oil.

Introduced into a hollow space existing on a side of the ceramicmeasuring cell 57 racing away from the separating diaphragm is a fillingbody 99, which serves the purpose of minimizing the free volume andconsequently the required amount of fluid.

In the chamber 98, the measuring cell 57 is surrounded on all sides bythe fluid. The fluid forms an isostatic mounting for the measuring cell57 which is completely free of restraint and in which the measuring cell57 is exposed to the same pressure on all sides. This mounting of theceramic measuring cell 57 improves still further the measuring accuracyof ceramic measuring cells, which is in any case very stable over a longtime in comparison with other measuring cells.

The electrical connection of the ceramic measuring cell 57 takes placeby means of contact pins 100 which are connected to theelectromechanical transducer, soldered onto the basic body and connectedto lead-throughs 101 led through the cover 95. On the cover 95, anelectronic circuit 102 for receiving the measuring signals of theelectromechanical transducer is connected to the lead-throughs 101.

For the case in which a relative-pressure measuring cell is used, thecover 95 has an additional lead-through 103, which is represented inFIG. 7 and through which a pressure supply line 105 is led right intothe ceramic measuring cell 57. A reference pressure, for example anambient pressure, is supplied via this pressure supply line 105.

In the case of the exemplary embodiments represented in FIGS. 4 and 7 aswell, the connection of the ceramic measuring cells 57 to the diaphragmseal 45 by means of inorganic materials makes polymer seals superfluousfor the pressure-resistant restraint of the measuring cells.Consequently, the very good long-term stability of the ceramicpressure-measuring cells is not impaired by organic seals.

1. A pressure sensor, having a housing, said housing defining a cavity;a diaphragm seal, comprising a diaphragm seal body, said diaphragm sealbody being connected to said housing, said diaphragm seal bodycomprising a first surface facing said cavity, a second surface facingaway from said cavity, and a passageway extending between said firstsurface and said second surface, a separating diaphragm on which apressure to be measured acts, said separating diaphragm being sealinglyattached to said second surface; a ceramic pressure measuring cell;having a ceramic base body and a ceramic pressure measuring diaphragm;and a metallic joint, wherein said pressure measuring cell is sealinglyfastened to said housing in said cavity by means of said metallic joint,the pressure measuring diaphragm being in fluid communication with saidpassageway.
 2. The pressure sensor of claim 1, wherein: said housingconsists of a material which has a coefficient of thermal expansionwhich is approximately equal to the coefficient of thermal expansion ofthe ceramic material of the ceramic pressure measuring cell.
 3. Apressure sensor, having a housing, said housing defining a cavity; adiaphragm seal, comprising a diaphragm seal body, said diaphragm sealbody being connected to said housing, said diaphragm seal bodycomprising a first surface facing said cavity, a second surface facingaway from said cavity, and a passageway extending between said firstsurface and said second surface, a separating diaphragm on which apressure to be measured acts, said separating diaphragm being sealinglyattached to said second surface; a ceramic pressure measuring cell;having a ceramic base body and a ceramic pressure measuring diaphragm;and an insert, said insert being mounted in said cavity, wherein: saidpressure measuring cell is fastened to said insert, said insertsurrounding said pressure measuring cell in a pot-like manner, and thepressure measuring diaphragm being in fluid communication with saidpassageway.